The present invention relates to an IPS (In Plane Switching) liquid crystal displaying apparatus by generating an electric field parallel to an array substrate to drive the liquid crystal. More particularly, the present invention relates to a construction of a highly bright liquid crystal displaying apparatus increased in aperture ratio by reducing influences of the leakage of electric field from a signal line, thereby reducing the light shielding area.
In an active matrix type liquid crystal displaying apparatus, an IPS system where the direction of the electric field to be applied on the liquid crystal is made parallel to the array substrate is mainly used as a method of obtaining a wider viewing angle (for example, see Japanese Unexamined Patent Publication No. 254712/1996). It is reported that this system removes almost all of the change in the contrast and the inversion of the gradation level in changing the viewing-angle direction (see, for example, AsiaDisplay, 95, page, 577 to 580 by M. Oh-e, and others).
A construction of one pixel of the conventional IPS liquid crystal displaying apparatus is depicted in FIGS. 43a and 43b. FIG. 43a is the plain view thereof. FIG. 43b is a sectional view taken along a line Axe2x80x94A of FIG. 43a. FIG. 44 is a circuit diagram showing an equivalent circuit of one pixel of the pixel electrode of an IPS liquid crystal displaying apparatus. FIG. 45 is a circuit diagram for illustrating the circuit of the IPS liquid crystal displaying apparatus. Referring to FIGS. 43a and 43b, reference numeral 1 denotes a glass substrate, numeral 2 denotes a scanning line, numeral 3 denotes a signal line, numeral 4 denotes a thin film transistor (TFT), numeral S denotes a driving electrode, numeral 6 denotes an opposite electrode, numeral 7 denotes an electrode for forming the storage capacitance, numeral 8 denotes common line, numeral 9 denotes a gate insulating film, numeral 10xe2x80x2 denotes a passivation film, numeral 11 denotes a liquid crystal, numeral 12 denotes a BM (black matrix), numeral 14 denotes a contact hole, numeral 15 denotes a source electrode, and numeral 16 denotes a drain electrode. Numeral 20 denotes an array substrate comprising glass substrate 1, a signal line 3, a driving electrode 5, an opposite electrode 6. Numeral 30 denotes an opposite substrate arranged opposite to the array substrate 20. Numeral 40 denotes a slit which is a gap between the signal line 3 and the opposite electrode 6, and numeral 50 denotes an opening. Referring to FIG. 44 and FIG. 45, the same reference numerals as those of FIGS. 43a and 43b depict the same parts or its equivalents as those of FIGS. 43a and 43b. 
The construction and operation of the conventional IPS liquid crystal displaying apparatus will be described according to FIGS. 43a and 43b, FIG. 44 and FIG. 45. Referring to FIG. 45, a plurality of grid shaped pixels encircled by the scanning line 2 and the signal line 3 can be made by crossing, at an approximately right angle between a scanning line 2 connecting the scanning line driving circuit 102 and a signal line 3 connecting the signal line driving circuit 101. A TFT (Thin Film Transistor) is provided at each intersection point between a signal line and a scanning line for forming the grid shaped pixel. Numeral 103 denotes a circuit for common lines.
This condition is shown by an equivalent circuit in FIG. 44. The TFT 4 is a semiconductor element having three electrodes of a gate electrode, a source electrode 15 and a drain electrode 16. The gate electrode is connected with a scanning line 2 extended from the scanning line driving circuit. The source electrode 15 is connected with the signal line 3 connected with the signal line driving circuit. The remaining drain electrode 16, connected with the driving electrode 5, drives the liquid crystal by an electric field caused between the driving electrode 5 and the opposite electrode 6. Numeral 7 denotes a storage capacitor for storing the electric charge between the driving electrode 5 and the opposite electrode 6. The construction of one pixel will be described in accordance with FIG. 43a and FIG. 43b. In a pixel formed through the crossing between the scanning line 2 and the signal line 3 are provided a driving electrode 5 for driving the liquid crystal layer, an opposite electrode 6 and a TFT 4. In the TFT 4 there are three electrodes. The scanning line 2 connected with the scanning line driving circuit shown in FIG. 45 is connected with the gate electrode of the TFT 4, so as to apply the scanning signal, the scanning line driving circuit outputs, upon the gate electrode of the TFT 4.
The signal line 3 connected with the signal line driving circuit is connected with the source electrode 15 of the TFT 4 to transmit the image signal the signal line driving circuit outputs. The drain electrode 16 of the TFT 4 is connected with the driving electrode 5 through a contact hole 14 as shown in FIG. 43a. In the same pixel, an opposite electrode 6 is provided to be engaged face to face with the driving electrode 5. The opposite electrode 6 is connected with the common line 8. The common line 8 is connected with each opposite electrode 6 provided in each pixel on the TFT array substrate 20.
The sectional construction of the picture section will be described in accordance with FIG. 43b. A driving electrode 5 and an opposite electrode 6 are respectively formed on the glass substrate 1. Although not shown in FIG. 43b, the scanning line 2 and the common line 8 are also formed in the same layer as that of the driving electrode 5 and the opposite electrode 6. The gate insulating film 9 is laminated on a glass substrate by covering the driving electrode, the opposite electrode, the scanning line and the common line, and the signal line 3 is formed on the gate insulating film 9. Although not shown in FIG. 43b, the storage capacitor forming electrode 7 is also formed in the same layer as that of the signal line 3. A passivation film 10xe2x80x2 is laminated further on the signal line 3, so as to form the TFT array substrate 20. The TFT array substrate 20 and the opposite substrate 30 is superposed. The IPS liquid crystal displaying apparatus is made with a liquid crystal 11 being sealed between the TFT array substrate 20 and the opposite substrate 30.
The IPS liquid crystal displaying apparatus is a system where the electric field is caused along the surface of the TFT array substrate 20 between the driving electrode 5 and the opposite electrode 6 provided on the TFT array substrate 20. Thus, the opposite substrate 30 is a no-electrode substrate having no electrode. On the opposite substrate 30 there is provided a black matrix BM 12 which is a light shielding film. Although not shown, the light leaked from a slit 40 of FIG. 43a is to be shielded with a back light, provided on the under side of the TFT array substrate, as a light source in FIG. 43b. 
An area surrounded by broken lines shown by 50, defining an opening per pixel, functions as a role of a window through which light passes with the back light as a light source. But the light from the back light is shielded by a driving electrode 5, an opposite electrode 6, a black matrix 12 and so on, thereby influencing upon the picture quality of the liquid crystal display. Thus, a problem is to reduce the ratio, in area, of the driving electrode 5, the opposite electrode 6, the black matrix 12 and so on to be occupied in the area of the opening 50.
The above description is given about the construction of the pixel of the conventional IPS liquid crystal displaying apparatus about FIGS. 43a and 43b, FIG. 44 and FIG. 45. The operation of the IPS liquid crystal displaying apparatus will be described. The gate electrode is provided in each pixel. The gate electrode of the TFT is connected with the scanning line 2. The source electrode 15 is connected with the signal line 3. The drain electrode 16 is connected with the driving electrode 5. Such a TFT 4 is a semiconductor switching element, which controls the driving operation of the liquid crystal of each pixel. When a scanning signal is applied, through the scanning line 2 from the scanning line driving circuit, upon the gate electrode of the TFT 4, all the TFT 4 of this horizontal line is respectively switched on.
When the gate electrode is switched on, the image signal transmitted from the signal line driving circuit flows to the drain electrode 16 by way of the source electrode 15 and is stored in the driving electrode connected with the drain electrode 16. Electric charge applied in the driving electrode 5 is stored with respect to the opposite electrode 6 and the gate electrode is turned on again. The electric charge of that time is stored before the new image signal electric charge is applied. The driving electrode 5 and the opposite electrode 6 function as a capacitor in that the electric charge is stored while the gate electrode is on, and the stored electric charge is held as it is when the gate electrode is turned off. The storage capacitance 13 shown in FIG. 44 increases the accumulating force of the capacitance. The storage capacitance 13 is formed by the vertical lamination of the storage capacitance electrode 7 and the common line 8 through the gate insulating film 9.
In the conventional IPS liquid crystal displaying apparatus shown in FIGS. 43a and 43b, between the signal line 3 provided in the side end portion of one pixel and the opposite electrode 6 formed in parallel to the signal line 3 is caused an electric field due to the potential difference between the signal line 3 and the opposite electrode 6. FIG. 46 is a view showing influences to be applied, upon the electric field to be caused between the driving electrode 5 and the opposite electrode 6, by the electric field caused between the signal line 3 and the opposite electrode 6 of the conventional IPS liquid crystal displaying apparatus, which has the TFT array substrate where the driving electrode 5 and the opposite electrode 6 are formed in the layer lower than the signal line 3. In FIG. 46, changes in the potential caused between the driving electrode 5 and the opposite electrode 6 is obtained as a simulator. In FIG. 46, the electric potential in the window upper portion or lower portion is calculated when a white window has been displayed in the half tone of the relative transmission factor 50%.
It is desirable to correctly drive the liquid crystal to have the driving electrode 5 between two opposite electrodes 6 so that the potential distribution is symmetrical around the driving electrode 5. It is found out from FIG. 46 that the potential distribution of an area near the signal line 3 of the opening 50 is subjected to the influences of the leakage of electric field caused between the signal line 3 and the opposite electrode 6, thus resulting in asymmetric potential distribution. The electric field is caused along the surface of the glass substrate 1, thus causing a problem like crosstalk. For example, when a white window is displayed in such black displaying as shown in FIG. 47, there prises a problem on the display called xe2x80x9clongitudinal crosstalkxe2x80x9d where the vertical luminance of the window portion changes with respect to the other black displaying portion.
An example in a case of a normally black mode (wherein the displaying becomes black with the voltage being not applied) will be described in FIG. 44. When such a window pattern in FIG. 47 is displayed, the same voltage as that of the opposite electrode 6 is applied during the selecting period of the black displaying portion 111 upon the signal line 3 of the pixels of the window and its upper and lower portions during the picture face, and a voltage necessary to the white displaying 113 is applied during the selecting period of the white displaying portion 111.
The voltage of a value where the absolute value of the electric potential value between the electrodes has been averaged by hour is applied upon the liquid crystal 11 effectively. Therefore, for example, when the black displaying and the white displaying are equal in the selecting period, the effective potential equal to the half tone display 112 is applied upon these pixels between the signal line 3 and the opposite electrode 6. At this time, the liquid crystal on the slit 40 between the signal line 3 and the opposite electrode 6 becomes a transmission mode by the electric field to horizontal to the glass substrate 1 to be caused between the signal line 3 and the opposite electrode 6. The electric field to be caused by the electrical potential difference between the signal line 3 and the opposite electrode 6 gives influences even upon the electric field between the driving electrode 5 and the opposite electrode 6, so as to change the liquid crystal of the black displaying portion into the transmission mode. As a result, the crosstalk is caused.
In order to prevent such longitudinal crosstalk from being caused, the leaking light transmitting through the slit 40 between the signal line 3 and the opposite electrode 6 is required to be shielded by the BM 12 formed on the opposite substrate 30 and to prevent the electric field, caused between the signal line 3 and the opposite electrode 6, from being interfered with the electric field between the driving electrode 5 and the opposite electrode 6 with the driving electrode 5 and the opposite electrode 6 spaced apart from the opposite electrode 6 of the side end portion on the side of the opening 50, and the signal line 3. When the driving electrode 5 and the opposite electrode 6 are separated from the signal line 3 to make larger the width of the opposite electrode 6 adjacent to the signal line 3, and the aperture ratio of the opening 50, namely, a portion to be occupied by an area where the area of the driving electrode 5 and the opposite electrode 6 and so on is subtracted from the area of the opening 50 with respect to the area of the opening 50 surrounded with broken lines in FIG. 43a, becomes smaller to make the picture quality worse. In order to develop the high picture quality liquid crystal displaying apparatus, it is necessary to shield the light, without reducing the aperture ratio, the electric field to be caused between the signal line 3 and the opposite electrode 6 adjacent to the signal line 3.
As clear from FIG. 43b, level of the surface of the passivation film 10xe2x80x2 which is an upper layer film of the array substrate 20 is not flat (level difference), and the gap between the surface of the passivation film 10xe2x80x2 and the opposite substrate 30 is not flat. Thus, uneven luminance is likely to be caused, causing the picture quality to worsen. The level difference provided makes not only the array substrate inferior due to cracking, but also disconnects the wiring on the array substrate due to the level difference portion in the manufacturing operation with a problem in improving the yield factor and reliability of the product.
Further, in accordance with the conventional IPS liquid crystal displaying apparatus, picture quality is deteriorated by light leaking transmitted from the slit 40, the light being emitted by a back light serving as a light sourse. In order to shield the leaked light, the black matrix 12 is provided on the opposite substrate 30. However, when the TFT array substrate 20 is superposed with the opposite substrate 30, there might be generated error. Then, the black matrix 12 has been formed in such a manner as to be somewhat larger with some margin for the purpose of taking the error into consideration However, there arises such a problem in which opening ratio is lowered when shieding effect is enhanced by making the black matrix 12 large.
The first object of the present invention is to solve the problems mentioned above, and to provide an IPS liquid crystal displaying apparatus causing electric field parallel to a glass substrate, the IPS liquid crystal displaying apparatus capable of improving shielding effect against electric field leaking from the signal line, making the opening wide (that is, making opening ratio high) by lowering the light shielding area. Further, the second object of the present invention is to provide a high quality IPS liquid crystal displaying apparatus in which cost for producing the apparatus is decreased by preventing the lines from disconnection thereby improving the yield factor.
The IPS liquid crystal displaying apparatus of the present invention comprises:
a TFT array substrate,
an opposite substrate opposed to the TFT array substrate and
liquid crystal interposed between the TFT array substrate and the opposite substrate,
wherein the TFT array substrate is composed of a glass substrate, a gate insulating film formed on the glass substrate, a passivation film formed on the gate insulating film, a plurality of scanning lines for transmitting a scanning signal, the plurality of scanning lines being formed on the glass substrate, a plurality of signal lines for transmitting an image signal, the plurality of signal lines being formed on the gate insulating film, a plurality of pixels arranged in grid like pattern by crossing the plurality of scanning lines with the plurality of signal lines, a plurality of TFTs implementing switching operation of the image signal on the basis of the scanning signals, a plurality of driving electrodes connected with the TFT, a plurality of opposite electrodes arranged in such a manner that each of the plurality of opposite electrodes is opposed to each of the driving electrodes, and a plurality of common lines for mutually connecting each of the opposite electrode of one of the plurality of pixels with the other one of the plurality of pixels,
wherein the TFT array substrate is formed on the passivation film, the passivation film being different from a layer provided with the driving electrode and the opposite electrode.
The IPS liquid crystal displaying apparatus of the present invention is provided with a driving electrode for driving the liquid crystal layer by causing the electric field parallel to the TFT array substrate face, the driving electrode being connected with the TFT, and an opposite electrode connected with a common line. At least the opposite electrode has a TFT array substrate formed on the passivation film, different from a layer where the signal line is formed.
The IPS liquid crystal displaying apparatus of the present invention has a TFT array substrate having an opposite electrode formed to cover one portion of the signal line or all the portion of the signal line.
The IPS liquid crystal displaying apparatus of the present invention has a TFT array substrate having an opposite electrode formed to cover one portion of the scanning line or all the portion thereof, having at least an opposite electrode in a layer different from the scanning line.
The IPS displaying apparatus of the present invention has a common line and a scanning line on the same layer, and a signal line provided on the gate insulating film.
The IPS liquid crystal displaying apparatus of the present invention has a TFT array substrate with the surface of the passivation film being approximately flat in shape.
The IPS liquid crystal displaying apparatus of the present invention has a light shielding means formed to have the signal line and the opposite electrode superposed.
The IPS displaying apparatus of the present invention has a TFT array substrate formed, to have for superposition in different layers, a TFT for switching the picture image signal in accordance with the scanning signal, a driving electrode for accumulating, while the switch of the TFT is off, the electric load stored when the switch of the TFT is on, and a storage capacitance increasing electrode for reinforcing the capacitance of the driving electrode.